1. Field of the Invention
The present invention relates to image processing to increase the resolution of an output image higher than that of an input image in an image having been subjected to pseudo halftone processing, such as a dither matrix.
2. Description of the Related Art
Conventionally, in an image forming device capable of producing a binary output to specify to print a dot or not, a halftone of a natural image etc. is reproduced by performing pseudo halftone processing, such as error diffusion and a dither matrix, on a multi-valued image. In the case where a halftone is reproduced by concentrating dots by a dither matrix, normally it is unlikely that texture becomes conspicuous even in the case where isolated points (dots) are regularly aligned. However, in the case where each dot is not so small, in particular, in a bright part close to white (low-density part) or in a dark part close to black (high-density part), there has been such a problem that dots are regularly aligned to be conspicuous, and therefore the image quality degrades inevitably for the multi-valued output with the same resolution.
It has been possible to cope with the problem of image quality degradation by increasing the output resolution. However, since as the resolution increases, the more storage capacity is required and the precision required for the device increases, and therefore it becomes extremely difficult to cope with the problem also technically, which causes a new problem of cost increase.
For example, in electrophotography, a development method in which a potential difference is made by laser irradiation and toner is attracted thereto is used. This is converted into a binary image (image in which a dot is printed or not) using a dither matrix to represent an image. In this case, the shape of a laser spot is circular and larger than a dot generated by a dither matrix, and therefore, in the case where positions neighboring one another are irradiated with the laser, there is a possibility that the laser spots overlap one another and the peaks of potential gather to produce a large peak suddenly. For example, it is known that in the case where a dot of 2×2 pixels is formed, the four spots gather and the peak of potential becomes large suddenly. Such a dot of 2×2 pixels is formed, for example, by simply enlarging a dot in order to match the resolution in the case where an image formed by 600 dpi is output to an electrophotographic device having an engine of 1,200 dpi.
In order to cope with this, a method is proposed, in which an isolated pixel is divided into smaller pixels while preventing the area thereof from changing in the case where an output is produced with an increased resolution, the divided pixels are arranged separate from one another, and thereby the gathering of the laser spots is lessened and dots are made less conspicuous (for example, see Japanese Patent Laid-Open No. H11-308461 (1999)).
However, with the method described in the above-mentioned Japanese Patent Laid-Open No. H11-308461 (1999), there is a case where the divided pixels are connected to one another in the case of a strong laser spot and the effect that dots are made less conspicuous is lessened. There is also a case where even though the divided pixels are not connected to one another, the dots exist close to one another, and therefore, the dots are viewed as one dot through the human eyes and are eventually conspicuous.